1. Field of the Invention
The present invention relates to a sheet feeder useful for various apparatus provided with a mechanism for feeding a sheet such as computer, copier, printer, facsimile equipment, word processor, typewriter etc.
2. Related Background Art
In the conventional sheet feeding mechanism used in the above-mentioned apparatus, a rotating driving source such as a motor has been used to drive rubber rollers, belts or the like by which the feeding of sheet is effected.
Another type of sheet feeding system known in the art is a vibration type of sheet feeding system as disclosed, for example, in Japanese Patent Application Laid-Open No. 59-177,243. In this sheet feeding system, a pair of elastic members are used to generate travelling vibration waves therein The elastic members are in pressure-contact with a sheet to be fed and apply to the sheet a feeding force derived from the travelling waves produced in the elastic members
FIG. 3 illustrates the principle of operation of the sheet feeding system as disclosed in the Japanese Patent Publication. The reference characters 11 and 12 in FIG. 3 denote a pair of elastic members in which travelling bending vibrations are produced as shown in the figure. A sheet 13 is sandwiched in the elastic members 11 and 12. A phase difference is set between the bending vibrations on the elastic members 11 and 12, whose phase difference is 180.degree. spacially. Therefore, the bending vibrations travel in such a manner that the convex portion, on the sheet side, of the bending of the elastic member 11 is always opposed to the convex portion, on the sheet side, of the bending of the elastic member 12. If a certain point on the elastic member is particularly examined at the time, it will be shown that the point moves describing an elliptic locus as indicated by two arrows in FIG. 3. Regarding the elastic member 11, when the bending vibration on it travels rightwardly, the point moves clockwisely describing an elliptic locus as shown in FIG. 3. Consequently, the direction of the movement of the point on the elastic member 11 is opposite to the travelling direction of the vibration. The same is applicable to the movement of a point on the other elastic member 12. From these movements of the points on both of the elastic members 11 and 12, there is produced a force for feeding the sheet 13. As indicated by an arrow, the direction of the sheet feeding is, therefore, opposite to the travelling direction of the bending vibration.
FIG. 4 is a perspective view of an example of the apparatus in which a sheet feeding force is produced in accordance with the principle described above.
Again, the reference characters 11 and 12 denote elastic members and 13 denotes a sheet to be fed. 14-1, 14-2, 15-1 (not shown) and 15-2 denote vibration members, 16 a pressing and supporting member, 17-1 and 17-2 side support plates and 18 a bottom plate.
The elastic member 12 is supported on the bottom plate 18 and the elastic member 11 is supported by the pressing and supporting member 16. The member 16 possesses some elasticity with which the sheet 13 is pressed against the lower elastic member 12 through the upper elastic member 11. In this manner, the elastic members 11 and 12 hold the sheet sandwiched in between them. In the manner described above, the elastic members 11 and 12 are brought into bending vibration. At the time, the convex portion of bending of the elastic member 11 is substantially opposed to the convex portion of the bending of the elastic member 12. As a result of it, there is produced a sheet feeding which can move the sheet 13 in a direction indicated by the arrow in FIG. 4. The arrow is shown as a double arrow which means that the direction of sheet feeding is reversible by changing over the travelling direction of the vibration.
The sheet feeding system as shown in FIGS. 3 and 4 has, however, some problems to be solved.
Although not shown in FIGS. 3 and 4, the vibration type of sheet feeder according to the prior art has generally a plurality of grooves and vibrating elements provided on the elastic members (vibration members) in order to lower the neutral plane of the thickness thereby achieving the desired effect of vibration. When a sheet is held and moved between such elastic members provided with a plurality of grooves and vibrating elements, the following problems arise:
(1) Thrust and speed obtainable for sheet feeding are not stable but variable depending upon the accuracy of alignment of two elastic members, especially alignment of the vibrating elements on one elastic member with the vibrating elements on the other elastic members. Because of it, the thrust and speed may be so dropped down that the desired efficiency of sheet feeding can not be obtained.
(2) Thrust and speed obtainable for sheet feeding are not stable but variable also depending upon the correspondence in phase of the travelling waves produced on two discrete elastic members. If the phase of the travelling wave produced on one elastic element does not correctly correspond to the phase of the travelling wave produced on the other elastic element, the thrust and feeding speed may be so dropped down that the desired efficiency of sheet feeding is no longer attainable.